The field of the invention pertains to fiber optic sensors instantaneously sensitive to pressure or stress in a manner that causes a beam of light to be reflectively modulated in response to changes in pressure or stress on the sensor.
In particular, in the automotive field, the ability to continuously monitor internal combustion engines for pressure fluctuations can significantly improve engine efficiency, performance, reliability and operating costs. Most importantly, the level of emissions can be reduced over the 100,000 mile effective life of the engine emissions control systems to be required by the United States Environmental Protection Agency. In addition, open and closed loop controls based on pressure information permit lean-burn engine operation, a wider tolerance to fuel octane and acceptance of alternative fuels.
Two combustion parameters, engine knock and misfire, have a particularly significant effect on overall engine performance. Combustion knock causes increased fuel consumption, reduced engine torque and engine deterioration if left uncorrected. Eventually severe damage such as perforated pistons can occur.
Misfire can result in catalyst damage and degradation that eventually cause vehicle exhaust emissions to no longer meet current or proposed emission standards. With a design 100,000 mile catalyst life, the failure to detect and correct misfire could result in operation of the vehicle for a lengthy period of time, possibly many years, with an ineffective catalyst.
The California Air Resources Board has recently proposed regulations which will require vehicles to be equipped with on-board emission monitoring systems. Such systems, in particular, will require misfire monitoring. The Environmental Protection Agency is also considering regulations to require such monitoring systems.
A low cost, reliable cylinder-selective combustion pressure sensor would permit knock and misfire detection separately for each cylinder. In addition to signalling the malfunction to the vehicle operator, a real time solid state engine control could adjust specific cylinder parameters to correct for the malfunction. The majority of the presently available or proposed knock and misfire detection techniques provide information that is not cylinder specific and therefore has limited utility for real time corrective controls. However, some recent patents disclose cylinder specific sensors. These sensors generally fall into two categories, luminosity detectors and pressure detectors.
U.S. Pat. No. 4,919,099 discloses a probe insertable into the engine cylinder combustion chamber. The probe includes a light conductive rod and fiber optic transmission bundle connected to an opto-electronic detector for instantaneous detection of the luminosity of the combustion gases within the combustion chamber. U.S. Pat. No. 5,052,214, in a similar manner, utilizes a fiber optic probe and transmission cable to sense and transmit the instantaneous luminosity to an opto-electronic detector. International Application Publication WO 89/11031 and European Application Publication EP-392-650-A also disclose optical luminosity probes for engine combustion chambers.
U.S. Pat. No. 4,781,059 discloses an optical fiber pressure sensor comprising a plurality of fibers to transmit light to the sensor tip and a second plurality of fibers to transmit light from the tip to an opto-electronic detector. The tip comprises a reflective diaphragm sensitive to pressure changes within the combustion chamber. U.S. Pat. No. 4,924,870 to applicant discloses an optical fiber pressure sensor tip comprising a single optical fiber. The single fiber carries dual light beams of differing wavelengths as input and the reflected return light beams. One wavelength serves as a reference signal that is reflected by an optical filter. The other wavelength passes through the optical filter and is reflected and modulated by a moveable diaphragm sensitive to pressure changes. This particular fiber optic sensor is of very small size, being intended for the measurement of intra-vascular blood pressure in human patients.
Two other patents to the applicant, U.S. Pat. Nos. 4,932,262 and 4,932,263, disclose a well having an optical fiber passing therethrough. A pressure sensitive membrane encloses at least a portion of the well. The underside of the pressure sensitive membrane includes an optical grating that couples with the wavelength of the light beam in the optical fiber so as to modify the light beam in response to pressure induced movement of the membrane. By making the sensor with techniques common to the manufacture of integrated circuits on chips, the sensor may be made small and rugged enough to locate on a spark plug in direct exposure to the combustion chamber of an engine.
A published paper co-authored by the applicant is entitled "Microbending Losses of Metal Coated Single Mode, Multimode, and Cladding-Free Fibers," Society of Photo-Optical Instrumentation Engineers, Vol. 985 Fiber Optic and Laser Sensors VI (1988) and discloses the test results of microbending various optical fiber constructions. The test results indicate the various attenuations of light beams as a function of microbending displacement of the fibers and wavelength of the light beams.
A second published paper co-authored by the applicant is entitled "A Fiber Optic Sensor for Combustion Pressure Measurement in a Washer Configuration," Society of Photo-Optical Instrumentation Engineers, Vol. 840 Fiber Optic Systems for Mobile Platforms (1987), and discloses a washer configuration for placement between a spark plug and engine cylinder head. Changes in combustion chamber pressure cause changes in the preload on the washer configuration. The washer configuration comprises upper and lower serrated washer halves with a continuous loop of optical fiber placed between the serrated washer halves. One end of the loop extends to a source of light and the other end of the loop extends to a photodetector.
In-cylinder pressure transducers are currently being considered for advanced engine control systems. Since cylinder pressure is the fundamental thermodynamic variable, it is used to determine a variety of engine parameters for closed-loop controls. In-cylinder pressure transducers are commonly used to determine apparent rate of heat release and indicated mean effective pressure (IMEP). Cylinder pressure history is also used to determine the best air/fuel ratio in closed-loop controls, thereby significantly increasing fuel efficiency and reducing emission levels of polluting gases. In addition, in-cylinder pressure sensors are best suited to adjust an engine's operating state on a cylinder-to-cylinder basis to minimize torque variability for lean-burn operations.
In the area of engine diagnosis, in-cylinder pressure sensors provide a direct and deterministic misfire detection, while indirect torque-fluctuation-based techniques are hampered by their inability to distinguish misfire from factors such as incorrect spark-timing and rough driving conditions.
Under recently enacted California Air Resources Board Regulations, on-board misfire detection, as a part of the overall legislation-mandated exhaust emission reduction efforts, will become an important and integrated part of electronic engine monitoring and control systems. Similarly, knock control is widely accepted as a major aspect of engine controls. In-cylinder pressure sensing is best suited to detect high frequency knock signals without being complicated by factors such as cylinder-to-cylinder variability, shock, vibrations, and signal phase-delays, plaguing externally mounted sensors.
The benefits of combustion pressure-based engine controls have long been recognized. However, commercial applications have been largely limited due to the lack of suitable pressure sensors that meet performance, size, and cost requirements. A major obstacle in developing viable and cost-effective combustion pressure sensors has been to overcome sensor performance degradation caused by adverse operating conditions, which include high combustion temperatures and strong electromagnetic interference (EMI). Long-standing and extensive efforts have been devoted to develop piezoelectric- and piezoresistive-type combustion pressure sensors, with limited successes in overcoming two inherent limiting factors. The first limiting factor is that sensing crystals cannot withstand high temperatures (above 300.degree. C.). Therefore, a transfer-pin is necessary to connect a pressure sensing diaphragm to the crystal, thereby locating the temperature-sensitive crystal away from high combustion temperatures. Such a construction is complicated in that temperature gradients presented to the mechanical assembly may induce response characteristics variations, such as hysteresis and other errors.
The second limiting factor is that signal conditioning electronics must be located at the sensor head to combat strong EMI effects. This routinely subjects the components to temperatures well over 125.degree. C., and complicates reliability and cost considerations. Along with the delicate sensing crystals, the electronic chip on the sensor head must be well shielded both electrically and thermally, leading to an enlarged sensor profile.
Consequently, these stand-alone sensors present difficulties in engine mounting as electronically controlled, multi-valve engines offer little available space. Further, caution must be exercised to avoid complications induced by ground-loops in an engine environment. So far, sensors based on the conventional technologies have not been able to meet overall cost, reliability, size, and high-temperature durability requirements for engine control applications.
Fiber-optic sensors for high-temperature pressure-sensing applications generally consist of fused-silica optical fibers embedded in metal-sensing fixtures, and are powered remotely by electro-optical modules. All components in the sensor heads are made of high-temperature-resistant materials that function without provisions for cooling or heat shielding. The sensors are electrically passive such that EMI and ground-loop problems are obviated. Although extensive fiber-optic sensor development work has been reported, the effort has been mainly toward the aerospace industry and more recently biomedical applications using low-cost disposable devices.
Adaptation of fiber-optic sensors to automotive applications is particularly challenging because: (1) The sensor must survive combustion temperatures up to 650.degree. C., (2) the sensor must provide accurate readings over operating pressure, temperature, vibration, and electromagnetic interference ranges encountered in the engine environment, (3) the sensor must maintain required reliability and accuracy up to over 100,000 miles of car operation, and (4) the sensor must meet the technical requirements at an extremely low cost.
Among various types of fiber-optic sensors, the most promising candidate for low-cost automotive applications is the simple intensity-modulated sensor. This sensor utilizes an optical fiber in front of a flexing diaphragm for optical reflection measurement of pressure-induced deflections. This sensor is disclosed in my parent patent application.
By employing this sensing principle coupled with a hermetically sealed sensor structure to eliminate diaphragm oxidation at high temperatures, and as discussed in my patent application, a sensor can operate at extreme combustion temperatures. However, the use of a flat diaphragm in this design can result in poor reliability over long-term exposure to high pressure and temperature cycling.
The primary reliability problem of flat diaphragm-based sensors for combustion pressure monitoring has to do with potential diaphragm fatigue resulting from continuous exposure to hundreds of thousands of pressure cycles. Since diaphragm deflections required in intensity encoded fiber optic designs are typically between 10 and 20 microns, high stress regions can be created at the diaphragm's center and at the clamped edge. While using a thicker diaphragm may result in reduced stresses and improved reliability, diaphragm deflection becomes too low for required optical signal changes.
The other problem of the diaphragm-based fiber optic sensors is related to the location of the laser welded area combining the diaphragm and the housing. The exposure of that area to extreme combustion temperatures over long time may cause sensor failure due to diaphragm yield.
Finally, flat diaphragm-based fiber optic sensor designs require the use of thick diaphragms for overpressure protection. For a typical overpressure range of 2000 psi, this means that diaphragm thickness may be increased as much as 50% reducing its deflection as much as 75% compared to a diaphragm designed for 1000 psi.
Thus, this disclosure describes a number of improvements in the metal diaphragm-based fiber optic combustion pressure sensor for improved reliability under conditions of long-term pressure and temperature cycling. The design also reduces the sensor's inaccuracies resulting from combustion flame kernel effect and provides for improved overpressure protection compared to a flat-disk based design.